The proposed research program will ascertain subjects from the population attending a Government-sponsored multiphasic testing centre. Married subjects aged 40-44 who have children and whose blood pressures fall above the upper 5th percentile for age and sex will be defined as "cases", and will be invited, with spouse and children, to attend for voluntary interview, examination, and venesection. Married controls with children (matched for age, sex, and country of birth) will be ascertained from those with a blood pressure below the lower 10th percentile for age and sex. One hundred "cases", 100 "controls" and corresponding spouses and children will be studied. Blood pressures, height, weight, skinfold, HLA phenotypes, other genetic markers, immunoglobulins, autoantibodies, immune complexes, cholesterol, triglycerides, electrolytes, glucose, uric acid, blood lead and 24 hour urinary excretions of sodium, potassium, creatinine and protein will be measured and related to information on diet, history of infectious disease, school history, family history and (for adults) psychological profile, work, smoking and drinking histories. The findings will be analysed to relate the distribution of blood pressure within and between families to the distribution of genetic, immunological, environmental and personal characteristics and, in particular, to test the hypothesis that some of the genetic and environmental causes of hypertension might act via immunological effector mechanisms.